Study finds Antarctic krill turn microplastics into nanoplastics

In the global war against plastics contamination, a groundbreaking Griffith University study has found Antarctic krill which ingest microplastics are able to turn them into nanoplastics through digestion.

The study, which formed the PhD research of Dr Amanda Dawson, was published in Nature Communications this week. The work was conducted under Associate Professor Susan Bengtson Nash’s Southern Persistent Organic Pollutants Program (SOPOPP) in collaboration with the Australian Antarctic Division, and has uncovered the ability of an Antarctic species to physically change ingested microplastics in a way not previously described.

“Despite a growing body of research, there are still considerable knowledge gaps regarding spatial patterns and abundance of microplastics in the marine environment,’’ Dr Dawson said.

“The phenomena of digestive fragmentation has never before been reported in other planktonic crustaceans despite the facts that many possess similar gastric mills and mouthparts designed for mechanical disruptions.”

The researchers also identified the potential for translocation (movement across biological membranes) to occur after an organism has physically altered the ingested plastics.

“This reveals a previously unidentified dynamic in the plastic pollution threat, with the implication that biological fragmentation of microplastics to nano plastics is likely widespread within most ecosystems,’’ Associate Professor Bengtson Nash said.

“As such, evaluating the harmful effects of plastic pollution must take into consideration not only the physical effects to the individual arising from macro and microplastic ingestion but also the potential cellular effects of nano plastics. Similarly, a biological role in plastic fragmentation will influence life cycle assessment of plastics in the environment.”

In the study the researchers exposed Antarctic krill to polyethylene (PE) microbeads along with an algal food source to determine the fate of microplastics ingested by a planktonic crustacean of high dietary flexibility and ecological importance. The krill were exposed to daily feeding either on a ‘high’ diet (80% PE and 20% algae) or ‘low’ (80% algae and 20% PE).

Whole krill were enzyme-digested after exposure to isolate the ingested microplastics as was faecal material collected through the experiment.

The researchers found all krill contained a mixture of whole PE microplastic beads and PE fragments. The fragments were, on average, 78% smaller than the original beads with some fragments reduced by 94% of their original diameter.

Whole beads were found in the stomach and midgut as well as faecal pellets. Exposure concentration played an important role in the ability of krill to fragment the PE beads where lower plastic concentration appeared to facilitate the krill’s capacity triturate (grind to a powder) plastic.

Krill contained significantly more whole beads when exposed to a high plastic diet than a low plastic diet. Faecal pellets also followed this trend.

At the beginning of each daily exposure, krill were efficient at fragmentation but as they ingested more beads the fragmentation efficiency decreased.

“Current contamination levels in the Southern Ocean are theoretically low enough to promote efficient digestive fragmentation by krill species, and in a global context, possibly for other zooplankton with sufficiently developed gastric mills,’’ said Dr Dawson.

The researchers observed microplastics within the esophagus, stomach, digestive gland and midgut of deceased krill and plastic was also visible in the stomach of live krill.

Their mandibles frequently had plastic fragments enmeshed in the grinding surface. The bulk of plastic maceration took place in the stomach and gastric mill, responsible for mechanically fragmenting food particles under usual feeding conditions.

Due to their mainly herbivorous diet, Antarctic krill have complex digestive systems.

The researchers did not examine the effects of digestive enzymes on microplastics so cannot rule out the possibility that digestive enzymes contributed to the fragmentation. While small food items pass through a filter into the digestive gland, large plastic fragments and full-sized beads were excluded from the digestive gland and directed to the midgut for excretion.

Krill being released back into the southern ocean. Photos supplied: Rob King/Australian Antarctic Division

Krill facts

Krill are ecologically important because they form the staple diet of many animals including seals, whales, fish, squid, penguins and other seabirds.

Krill feed on phytoplankton but regularly prey on other zooplankters. They filter feed by forming a feeder basket through water is passed. Food particles are retained on the basket and then transported to the mandibles for mastication. At the base of the esophagus, the mandible is equipped with a cutting and grinding surface. Food is then directed through the short esophagus into the stomach and gastric mill where it is mixed with digestive enzymes for further mastication.